1. Field of the Invention
The present invention relates in general to the use of diesel fuels in cold climates. More specifically, the present invention is directed toward the use of diesel fuels derived from a Fischer-Tropsch synthesis, diesel fuel pour point depressants, and heated fuel delivery systems to improve the performance of diesel fuels in cold climates.
2. State of the Art
Diesel fuels are consumed in virtually every country of the world. Although many of these countries are only subjected to cold climates during their winter seasons, others experience low temperatures year around. It can be a challenge to formulate diesel fuels for cold climates, especially if the objective is to stock only one type of fuel for use during both summer and winter seasons.
The challenge of low temperature operability of the middle distillate fuels stems from the fact that a typical fuel contains paraffin waxes that may precipitate out of solution if the fuel is cooled to a sufficiently low temperature. Paraffin wax is a predominantly straight chain paraffin having the general formula CnH2n+2, where the number of carbons in the molecule is typically greater than about 20.
As a diesel fuel is cooled, it reaches a temperature at which it is no longer able to keep its waxy components in solution. The temperature at which the wax begins to precipitate is known as the “cloud point” because wax crystals become visible as a suspension of small particles, imparting a cloudy appearance to the fuel. When this happens, solid wax particles can plug various elements of a fuel delivery system, most notably the fuel filters. This is not surprising, since fuel filters are designed to remove particulate solids such as grit and other debris that may potentially damage delicate engine parts such as the fuel injectors. Thus, a low cloud point is desirable if one wishes to achieve a steady and uninterrupted flow of fuel through the delivery system.
If the fuel is cooled below the cloud point, more wax can precipitate. At some temperature the viscosity of the fuel increases to a point where the fuel ceases to flow through the fuel lines, and a temperature that is approximately the “pour point” of the fuel has been reached. The pour point may also be a rough indicator of the temperature at which fuel will congeal in the fuel tank. Either of these two situations can be troublesome, if not disastrous, since an interruption in the fuel supply to an operating engine will cause it to cease functioning.
Cloud point and pour point values may be considered simultaneously to suggest a type of cold-climate specification. Typically, the difference between the cloud point and the pour point is less than about 5° C., where the cloud point is the higher of the two temperatures. While some fuel systems become plugged at the cloud point temperature, others can operate several degrees below the cloud point before plugging debilitates the system. This is because low temperature filterability depends on the size and shape of the wax crystals suspended in the fuel, and not merely on whether or not they are present.
Another challenge to contend with in cold geographic locations is getting an engine started in the first place. When attempting to start a cold engine, the heat of compression of the fuel within the combustion chamber is the only energy source available to heat the fuel to a temperature where it can spontaneously ignite (about 750° F.). Initially the walls of the combustion chamber function as a heat sink, rather than a heat source, since they are at a cold ambient rather than hot operating temperature. Furthermore, since the cranking speed of the engine is less than the operating speed, the compression of the fuel is slower initially, allowing more time for the fuel to lose heat to the chamber walls.
The ability to start a cold engine is related to the cetane number of the fuel, which is a measure of the tendency of fuel to combust spontaneously. In the cetane number scale, high values represent fuels that ignite readily, and thus high cetane number fuels perform better in diesel engines. Typically a minimum cetane number of 40 is required to ensure adequate cold starting performance, although higher number are desirable. When ambient temperatures are below freezing, starting aids may be necessary regardless of the cetane number of the fuel.
In addition to concerns about diesel fuel performance in cold-climate situations, there is mounting concern about excessive emissions from diesel engines. Emissions from diesel engines can be reduced if the sulfur content of the fuel is reduced to a level of about one part per million (ppm). Emissions may also be reduced if the aromatic content of the fuel is less than about one weight percent.
One of the techniques available for providing low emission fuels is to produce them from the products of a Fischer-Tropsch process. A Fischer-Tropsch synthesis is a process whereby a starting material called synthesis gas (or “syngas”), which comprises carbon monoxide and hydrogen, is converted to a mixture of long chain hydrocarbons comprising olefins, paraffins, and alcohols. The reaction may be considered a hydrogenative oligomerization of carbon monoxide in the presence of a heterogeneous catalyst, and the reactions have been described by S. Matar and Lewis Hatch in Chemistry of Petrochemical Processes, 2nd Ed. (Gulf Professional Publishing, Boston, 2001), pp. 121-126.
The Fischer-Tropsch process provides a product that is low in both sulfur and aromatics. Thus, from the standpoint of emissions, the products of the Fischer Tropsch process are ideal. Unfortunately, fuels derived from this source also contain normal paraffins in the form of waxes in the diesel boiling range that solidify at cold temperatures.
To optimize a Fischer-Tropsch fuel for cold climate use, it may be necessary to remove most if not all of the paraffins, especially the highest boiling ones. Normal paraffins are typically treated in an isomerization process that converts them into branched paraffins. However, this conversion is not completely selective, and some of the normal paraffins are converted into light by-products that cannot be blended into the diesel product without compromising the safety of the fuel by simultaneously lowering the fuel's flash point. An alternative solution is to reduce the end point of the diesel fuel, which excludes high boiling normal paraffins by “terminating” the distillation before they have a chance to distill over into the product. End point lowering and conversion techniques for decreasing the wax content are not always desirable as solutions to the wax problem, however, because each of these techniques reduces the yield of the product and hydrocarbon resources are becoming scarce.
What is needed is a diesel fuel designed for use in cold geographic locations and cold climate conditions that may optionally be used in warm weather as well. Such a fuel will have a low sulfur and aromatic content to reduce emissions, and a high cetane number for combustibility. The art lacks a cold climate fuel whose paraffin wax content can be tolerated such that the fuel may be produced in higher yields than otherwise would have been possible.